Reynolds Number Calibration Coefficient for Multi-Hole Pneumatic Probes

The calibration of an additively manufactured cone-shaped five-hole probe is presented for subsonic and transonic flow. The probe has a thermocouple and an additional base pressure hole for transonic measurements. The Reynolds number is varied independently of the Mach number by adjusting the pressure level in a closed test section in three steps. Overall, a Reynolds number range from 3,000 to 20,000 is covered. A significant Reynolds number effect is observed for the pressure coefficients. More than 1,100 test points not contained in the calibration data are evaluated and measurement deviations calculated. The points are distributed over six pressure levels and four Mach numbers. Measurement deviations increase with increasing Reynolds number differences between the calibration and the test points. The Mach number measurement in transonic flow using the base pressure hole is particularly sensitive to the Reynolds number. The root mean square error of the Mach number increases by up to factor of 15 if Reynolds number similarity is not maintained. This demonstrates the importance of Reynolds number similarity when calibrating probes with a base pressure hole. The effect on the flow angles is relatively low with the exception of the radial flow angle in transonic flow. Here, the Reynolds number effect reaches a maximum of 1 deg. A calibration coefficient is proposed that decreases measurement deviations by allowing interpolation between calibration data at different Reynolds numbers and can easily be introduced to most evaluation schemes.